1. Field of the Invention
The present invention relates to a process for separating non-lithium hydroxide solid compounds in which gaseous hydrogen sulfide is bubbled through an aprotic solvent containing solid lithium hydroxide and solid compounds other than lithium hydroxide (namely, non-lithium hydroxide solid compounds) to convert lithium hydroxide into lithium hydrosulfide soluble in the aprotic solvent.
2. Description of the Related Art
In processes for producing a polyarylene sulfide resin (hereinafter, referred to as PAS) using a lithium compound (e.g., U.S. Pat. No. 5,635,587), an expensive lithium compound must be recycled. Lithium is dissolved as lithium chloride in a polymerization reaction solution of PAS, and is recovered in the form of a crystal of lithium hydroxide by adding an equivalent-molar or more sodium hydroxide to lithium chloride. However, since sodium chloride in equivalent molar to lithium hydroxide precipitates as a crystal simultaneously, if lithium hydroxide is recycled without any treatment to the polymerization system of PAS, sodium chloride insoluble in an aprotic solvent used as a polymerization medium is incorporated in the PAS resin. Therefore, there is a need to separate lithium hydroxide and another solid compound (namely, non-lithium hydroxide solid compound such as sodium chloride and the like) presence on the aprotic solvent obtained by treating the polymerization reaction solution of PAS with sodium hydroxide.
As this process, there are known, (1) a process in which a large amount of water is added to a system composed of lithium hydroxide, other solid compounds and an aprotic solvent, and separation is conducted by utilizing a difference in degrees of solubility into water of the solid compounds, and (2) a process in which gaseous hydrogen sulfide is bubbled through an aprotic solvent containing lithium hydroxide and other solid compounds, and hydrogen sulfide and lithium hydroxide are reacted to synthesize a complex soluble in the aprotic solvent (e.g., N-methyl-2-pyrrolidone (hereinafter, may abbreviated as NMP)) which is separated (U.S. Pat. No. 5,635,587).
However, in the process (1), since the difference in degrees of solubility of lithium hydroxide and sodium chloride in water is small, separation efficiency is extremely poor. In addition, there is also a problem of a large amount of water remaining in the system. Further, in examples specifically disclosed as the process (2) of bubbling of gaseous hydrogen sulfide, there are disclosed a solid-liquid separation operation in which a reaction mixture after bubbling of gaseous hydrogen sulfide is poured on a glass filter kept at 130xc2x0 C. to be filtrated under reduced pressure, and an operation in which the residue out he filter is washed using NMP at 130xc2x0 C. However, since the S (sulfur)/Li (lithium) ratio (molar ratio) in the reaction system after bubbling of gaseous hydrogen sulfide is 0.65, it is hypothesized that a fair amount of lithium sulfide (Li2S) has been produced. Further, in solid-liquid separation, a filtration operation under reduced pressure, which tends to produce lithium sulfide, is adopted. Since lithium sulfide is insoluble in NMP, the problem of recovering a loss of lithium compounds is not supposed to be solved completely even if the above-described solid-liquid separation operation and washing operation are carried out at 130xc2x0 C.
The object of the present invention is to provide a process in which gaseous hydrogen sulfide is bubbled through an aprotic solvent containing solid lithium hydroxide and solid compounds other than lithium hydroxide (namely, non-lithium hydroxide solid compounds) to convert lithium hydroxide into lithium hydrosulfide soluble in the aprotic solvent before separation of the non-lithium hydroxide solid compounds by solid-liquid separation, wherein the recovering loss of lithium compounds is reduced.
The present inventor has intensively studied the above-described problems, and resultantly found that it is possible, in the above-described process for separating non-lithium hydroxide solid compounds, that lithium hydroxide is converted by using gaseous hydrogen sulfide into lithium hydrosulfide soluble in an aprotic solvent, solid-liquid separation is conducted, then, the separated crystal cake is washed with an aprotic solvent to recover, effectively, lithium hydrosulfide contained in the crystal cake, and completed the present invention based on this finding. The summary of the invention is as follows.
[1] A process for separating a non-lithium hydroxide solid compound in which gaseous hydrogen sulfide is bubbled through an aprotic solvent containing solid lithium hydroxide and a solid compound other than lithium hydroxide (namely, non-lithium hydroxide solid compound) to convert lithium hydroxide into lithium hydrosulfide soluble in the aprotic solvent before separation of the non-lithium hydroxide solid compound by solid-liquid separation, wherein the separated crystal cake is washed with the aprotic solvent to recover lithium hydrosulfide contained in the crystal cake.
[2] The process for separating a non-lithium hydroxide solid compound according to [1], wherein the solid-liquid separation is conducted under the condition in which the de-hydrogen sulfide reaction of lithium hydrosulfide is suppressed and at a temperature of the liquid separated at 50 to 150xc2x0 C.
[3] The process for separating a non-lithium hydroxide solid compound according to [1] or [2], wherein at least a part of the washing liquid (namely, aprotic solvent) used for washing of the crystal cake separated is recycled to a hydrosulfurization step of lithium hydroxide or a washing step of the crystal cake.